Dielectric relaxation spectroscopy (DRS) is attractive because of its ability to probe particle properties and interactions without the use of chemically attached dyes, labels, or markers. However, the widespread use of DRS as a sensor platform has been limited, in part, because of problems associated with interfacial polarization and surface regeneration of electrodes.
Most DRS and impedance spectroscopy techniques rely on immobilizing capture molecules onto electrode surfaces and/or monitoring impedance characteristics using electrodes in contact with test chamber solutions. A disadvantage of these techniques is interfacial polarization, which occurs at the fluid-electrode interface. The restriction of charge transfer at the interface between electrically dissimilar materials (e.g. metal electrodes and buffer solutions) causes charge accumulation at the interface. The free charge carrier concentrations and associated carrier mobilities of the two (or more) dissimilar materials create a discontinuity in charge carrier concentration. The energy loss due to the relaxation of interfacial polarization has been classified as β-dispersion. Although changes in capacitance resulting from binding to capture molecules immobilized on electrode surfaces and β-dispersion can be used as a sensor transduction mechanism, interfacial polarization generally serves to interfere with the measurement of other forms of dielectric relaxation.
Another disadvantage of electrode contact with test chamber solutions is surface regeneration. The requirement for surface regeneration can occur over both short- and long-term time scales. In the short-term, non-specific binding of molecules in the test chamber, whether related to the analyte under investigation or simply present in the vehicle solutions, can irreversibly interfere with DRS measurements. Typically, electrode plates eventually foul, requiring system recalibration and/or regeneration. In some cases, electrodes must be recalibrated or regenerated before each measurement. Typical DRS measurement setups can also exhibit drift due to the harsh conditions required for surface regeneration. In the long term, the eventual loss of capture molecules or other surface structures that convey sensor specificity, or corrosion of the electrode material itself, often necessitate replacement of the sensor.
Dielectric relaxation spectroscopy can be used to measure properties of various materials, including liquids and gels. For example, DRS can be used to gain information about the size, structural characteristics, and electronic characteristics of an analyte. In some implementations, DRS can be used in sensor techniques, including being used as a biosensor.
Biosensors are broadly defined as devices which are capable of transducing biochemical events into measurable electrical or optical signals. The transducer is a feature of many biosensor designs, and many different transducing schemes have been proposed. FIG. 1 depicts a biosensor where biological detector molecules, such as antibodies or antigens, are immobilized onto a transducer surface.
A summary of the characteristics of two immunological techniques is presented in FIG. 2. Immunosensors typically possess similar sensitivity, specificity, and accuracy to well established immunologically based assays such as enzyme linked immunosorbent assays (ELISAs), with the added advantage of being rugged and providing the results in real-time and on-line. A specific example of a biosensor with immobilized immunospecies on its surface is the capacitance affinity sensor (CAS), described in U.S. Pat. No. 4,769,121.
The CAS utilizes the field produced by a parallel plate capacitor to measure changes to its biochemically active layer, where the detector molecules are located. The CAS, like most immunosensors, requires periodic surface regeneration. Surface regeneration has been recognized as a source of drift in these sensors's output signal, and can severely limit their longevity.
Typical biosensors developed to date rely on biological binding events occurring on or near the surface of an optical waveguide or an electrode. Biological molecules immobilized on surface materials must typically endure harsh chemical environments when the surfaces are regenerated between biosensing measurements, and in some cases the entire biologically activated surface must be replaced before every measurement.
One mechanism that has been used in biosensor applications is agglutination. One example of an agglutination-based immunosensor is the latex agglutination test (LAT), based on immuno-specific interactions. A common commercially available LAT that is sensitive, easy to use and inexpensive is the First Response® pregnancy test manufactured by Armkel LLC (Princeton, N.J.). LAT's typically require a fluid sample to be placed onto a surface (often white paper) which has been pre-loaded with antigen- or antibody-coated microspheres. Biological species in the fluid sample either cause or inhibit agglutination of biologically coated microspheres so that a visual, qualitative determination can be made about the presence of the biological species. However, results are largely qualitative and, as configured above, are generally designed for single-use applications.